Electric contacts



United States Patent ELECTRIC CONTACTS Roland W. Smith and Albert Rose, Princeton, NJ.,

assignors to Radio Corporation of America, a corporation of Delaware I No Drawing. Application April 30, 1953 Serial No. 352,298

8 Claims. (Cl. 29- 195) This invention relates to electric contacts and more particularly to novel ohmic electric contacts to N- type semiconductors and insulators, and methods of making such contacts.

Photoconductor and semiconductor elements are prepared by connecting one or more leads to the active material in the element. While these connections, or contacts, may not affect the electrical characteristics of the active material itself, they do affect the over-all electrical characteristics of the element.

Metals, such as silver, copper and gold, are often used as materials for making contacts. These materials appear to form barriers at the interface between the contact material and the active material and, as aresul't, do not freely supply electrons to the active material of the element. The contact appears to have an abnormally high resistance in one or both directions of current flow. Furthermore, spurious voltages are generated by light at the contact interface and relatively large noise currents are produced in the Contact. All of these effects interfere with the efficient operation of the element.

An ohmic contact, on the other hand, appears free of barriers and barrier effects and only the volume resistances of the materials appears to oppose the flow of current through the contact. Noise currents are markedly lower and spurious light-generated voltages are not observed. In the absence of these adverse eifects, the contact freely supplies a large reservoir of electrons to the active element.

At times, prior art contacts made of metals such as mentioned above, appear to be ohmic in nature while in fact they are not. If a voltage higher than the breakdown voltage of the barrier is applied to such a contact, the current will vary according to Ohms law. However,'examination of these contacts at lower voltages and currents reveals that barriers do exist and that the contact is not ohmic. Where low-voltage, high sensitivity elements are required, an ohmic-type contact is necessary. In the case of photoconductive elements, for example, where very small photo-currents are allowed to pass, the use of ohmic-type contacts decreases the over-all resistance of the element, decreases noise currents produced at the contact, and decreases spurious voltages generated by light at the contact. The net eifect is to increase the efficiency and photosensitivity "of the element.

Ohmic contacts to semiconductors have been made by several well-known processes in many fields of the electric and radio arts. However, these processes usually require either (1) that the "semiconductor be formed directly upon the surface of the contact, or (2) that the contact material be diffused and alloyed into the semiconductor material. Both of these processes are expensive and diificult to control. m

It is, therefore, an object'of this invention to provide ohmic-type electric contacts for connections to cadmium sulfide or cadmium selenide semiconductors and insulators.

Another object of this invention is to provide -,ohmic- 2,916,810 Patented Dec. 15, 1959 .2 type electric contacts for "connections to cadmium sulfi'de or cadmium selenide semiconductors and insulators that does not require the semiconductor or insulator to be formed on the surface of the contact.

Another object of this invention is to provide ohmictype electric contacts for connections to cadmium sulfide or cadmium selenide semiconductors and insulators that do not require the contact material to be alloyed and diffused into the semiconductor or insulator material.

A 'further object ofthis invention is to provide an electric contact that generates only small noise currents at the Contact area.

Another object of this invention is to provide an electric contact for a photoconductor that is free from spurious voltages generated at the contact area due to the presence of light.

In general, this invention consists of the use of an electrode composed of a metal or an alloy of metals containing at least 50% of at least one metal selected from the group consisting of indium, gallium, tin, lead, magnesium and cerium, in intimate contact with the surface of a cadmium sulfide or cadmium selenide semiconductor or insulator body. The method of this invention is to form a Contact electrode having one of the above mentioned To make an ohmic contact to a cadmium sulphide crystal, a piece of indium metal is formed into an electrode of a desired shape. This may be done by any of the commonly known methods, for example, casting, rolling, punching, and stamping. A surface of the electrode so formed is pressed against the surface of a cadmium sulphide crystal so that the two surfaces are in intimate physical contact with each other. In order to facilitate good intimate physical contact between the surfaces, the electrode may be warmed for a fraction of a minute in a non-oxidizing atmosphere when the electrode is applied to the crystal. Electric currents passed through the resulting contact are linear at least in a range of voltages from 0.01 volt to 50 volts and currents at least up to 10 amperes per square centimeter. Noise currents observed are less than 1% of noise currents observed when silver contacts are used.

Example 2 Since indium melts at about C., heating as carried out in Example 1, often melts the indium and the indium forms a ball. To facilitate getting a good contact to a cadmium sulphide crystal, indium is coated on at least one side of a thin sheet of a soft metal, for example, nickel. The coated sheet is now formed into an electrode, and the coated surface of the resulting electrode applied to the surface of the cadmium sulphide crystal as described in Example 1. Also, Example 2 allows the use of a cheaper metal to serve as a base for the more expensive indium, without affecting the electrical properties of the contact.

Good Contacts can be obtained by using electrodes composed of a metal or an alloy of metals selected from the class consisting of indium, gallium, tin, lead, magnesium and cerium. These metals are believed to have low work functions. Pure indium and pure gallium or alloys thereof are the preferred materials out of which to form the electrode. Since gallium melts at about 30 C. and indium melts at about 155 C., the electrodes made of these metals can be brought into intimate contact with the active material in the element with the very smallest amount of heat and pressure. Tin, lead, magnesium and cerium are good contact materials also but, since their melting points are higher than indium or gallium, they are harder to work with. One or more of the abovementioned metals may be alloyed with other metals provided the other metals do not exceed 50% of the composition. For example, good contacts result from mixtures of bismuth and indium or gallium, or from mercury and indium or gallium. Alloys can be prepared by any of the well-known methods, for example, melting the con stituents together to form a solution of metals.

The electrode may be formed to a desired shape by any of the commonly known methods, for example, rolling, punching and stamping. The electrode in its simplest form is a single composition that has been formed to a desired shape. Alternatively, the electrode material may be coated on some other material that will serve as a base. For example, sheet nickel having a layer of indium and gallium on one or both sides and suitably formed makes a good electrode. Similarly, sheet nickel having a layer of indium on one or both sides and then a layer of gallium upon the layer of indium, and suitably formed, makes a good electrode.

After the electrode is formed, the surface of the electrode is applied to the surface of the semiconductor or insulator body. All that is necessary is that the two surfaces are in intimate physical contact with one another. If the electrode material is soft enough, merely placing the two surfaces against one another with the slightest pressure will effect a good ohmic contact. In other cases, pressure and heating are used to facilitate intimate physical contact between the surfaces. If heating is necessary, a non-oxidizing atmosphere will facilitate the operation. After the contact is made, the heat and pressure are removed. While heating may be used to obtain good contacts, it should be clear that it is used for the purpose of making intimate physical contact between electrode and body surfaces, and that it is not for the purpose of diffusing the electrode material into the body of the semiconductor or insulator. It is believed that no diffusion takes place. When the electrode is removed from the body after a previous contact has been made, there is no sign of the previous contact nor does a subsequent contact prefer the previous contact area.

A good ohmic contact may also be obtained by forming an electrode directly on the semiconductor or insulator body by several well-known methods. For example, the electrode may be formed by evaporating, sputtering or spraying the electrode material on the body.

Electrodes made and connected according to the present invention make good ohmic contacts to bodies of material that conduct electric current by negatively charged carriers or electrons, as opposed to conduction by positively charged carriers or holes. These materials may be insulators, semiconductors or photoconductors depending on their resistivities and photosensitivities. They are designated as N-type in this application to indicate the nature of the current carriers. Cadmium sulphide is classed as an insulator when its resistivity is greater than ohm-cm, as a semiconductor when its resistivity is less than 10 ohm-cm., and also as a photoconductor when it is photosensitive. These three forms of cadmium sulphide illustrate an N-type insulator, an N-type semiconductor and an N-type photoconductor to which ohmic contacts can be made by the method of this invention. Other materials that occur in these three forms and to which ohmic contacts can be made by the method of this invention are cadmium selenide and zinc oxide.

There has thus been described novel ohmic electric contacts and methods for making them. Although specific embodiments of this invention have been described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the spirit and scope of this invention.

What is claimed is:

1. An electrical device comprising a body of material selected from the class consisting of cadmium sulphide and cadmium selenide and at least one electrode composed of a second material selected from the class consisting of metals and alloys of metals, said second material containing at least 50% of at least one metal selected from the class consisting of indium, gallium, tin, lead, magnesium and cerium, the surface of said electrode being and remaining permanently in intimate physical contact and in ohmic electrical contact with the surface of said body without the external application of pressure.

2. An electrical device comprising a body of material selected from the class consisting of cadmium sulphide and cadmium selenide and at least one electrode composed of a second material selected from the class consisting of metals and alloys of metals, said second material containing at least 50% of indium, the surface of said electrode being in intimate physical contact with the surface of said body.

3. An electrical device comprising a body of material selected from the class consisting of cadmium sulphide and cadmium selenide and at least one electrode composed of a second material selected from the class consisting of metals and alloys of metals, said second material containing at least 50% of gallium, the surface of said electrode being in intimate physical contact with the surface of said body.

4. An electrical device comprising a body of material selected from the class consisting of cadmium sulphide and cadmium selenide and at least one electrode composed of indium, the surface of said electrode being in intimate physical contact with the surface of said body.

5. An electrical device comprising a body of material selected from the class consisting of cadmium sulphide and cadmium selenide and at least one electrode cornposed of gallium, the surface of said electrode being in intimate physical contact with the surface of said body.

6. An electrical device comprising a body of material selected from the class consisting of cadmium sulphide and cadmium selenide and at least one electrode comprising a thin sheet of a soft metal formed to a desired shape and coated on at least one side with a second material selected from the class consisting of metals and alloys of metals said second material containing at least 50% indium, said coated surface being in intimate physical contact with the surface of said body.

7. An electrical device comprising a body of material selected from the class consisting of cadmium sulphide and cadmium selenide and at least one electrode comprising a thin sheet of a soft metal formed to a desired shape and coated on at least one side with a second material selected from the class consisting of metals and alloys of metals said second material containing at least 50% gallium, said coated surface being in intimate physical contact with the surface of said body.

8. An electrical device comprising a body of material selected from the class consisting of cadmium sulphide and cadmium selenide and at least one electrode comprising a thin sheet of nickel formed to a desired shape and coated on at least one side with a second material selected from the class consisting of metals and alloys of metals said second material containing at least 50% indium, said coated surface being in intimate physical contact with the surface of said body.

References Cited in the file of this patent 

6. AN ELECTRICAL DEVICE COMPRISING A BODY OF MATERIAL SELECTED FROM THE CLASS CONSISTING OF CADMIUM SULPHIDE AND CADMIUM SELENIDE AND AT LEAST ONE ELECTRODE COMPRISING A THIN SHEET OF A SOFT METAL FORMED TO A DESIRED SHAPE AND COATED ON AT LEAST ONE SIDE WITH A SECOND MATERIAL SELECTED FROM THE CLASS CONSISTING OF METALS AND ALLOYS OF METALS SAID SECOND MATERIAL CONTAINING AT LEAST 50% INDIUM, SAID COATED SURFACE BEING IN INTIMATE PHYSICAL CONTACT WITH THE SURFACE ON SAID BODY. 